Resource utilisation and reproductive biology of syngnathid fishes in a seagrass-dominated marine environment in south-western Australia

Syngnathid species were sampled during the day and night in each season from summer 1996/97 to summer 1997 /98 in five habitat types in a ca 90 km2 area, referred to as the Owen Anchorage region, located on the lower west coast of Australia. Three seagrass habitats, dominated by Amphibolis griffithii, Posidonia sinuosa or Posidonia coriacea, and a bare sand habitat were at depths of 4-9 m. While A.griffithii and P. sinuosa each formed dense mono-specific meadows, P. coriacea occurred in sparse clumps surrounded by areas of bare sand and containing patches of the seagrass Heterozostera tasmanica. The deep habitat, which comprised mainly bare sand with isolated patches of H. tasmanica and Halophila ovalis, was at depths of 12-16 m. Each of the above five habitat types were sampled using a small beam trawl (1 m wide and with 1 mm mesh in the cod-end) and a large beam trawl (2.5 m wide and with 10 mm mesh in the cod-end). Syngnathids were also sampled during the day and night in each season between autumn 1997 and summer 1997 /98 using the small trawl in P. sinuosa in nearshore waters (ca 2 m deep) at sampling regions located at Garden Island and Safety Bay to the south of Owen Anchorage. The most abundant syngnathid species in the study area were spotted pipefish Stigmatopora argus and wide-bodied pipefish Stigmatopora nigra. In the Owen Anchorage region, the adults and juveniles of S. argus mainly inhabited P.sinuosa and P. coriacea, respectively, and this syngnathid species was more abundant in P. sinuosa at this region than in meadows of the same seagrass species in nearshore waters at Garden Island and Safety Bay. In contrast, S. nigra was found throughout its life cycle predominantly in P. coriacea and deep habitat, and was consistently more abundant in the former than in the latter habitat. This species was never found in P. sinuosa at the Owen Anchorage region. The short­snouted seahorse Hippocampus breviceps occurred mainly in P. sinuosa and P. coriacea, while long-snouted pipefish Vanacampus poecilolaemus were found almost exclusively in P. sinuosa. It is suggested that S. argus and S. nigra remain in habitats that best enable them to remain inconspicuous to predators by mimicking seagrass leaves. Thus, the movement of S. argus from the narrow-leaved P. coriacea to the broad-leaved P. sinuosa as it increases in size would enable the adults of this species to remain better camouflaged than if they had remained in the former habitat. Since the adults of S. nigra are smaller than those of S. argus, they are able to remain concealed among the narrow leaves of P. coriacea and H. tasmanica without undergoing a size-related shift in habitat. The diets of the 12 most abundant syngnathid species in the study area were dominated by a small number of crustacean taxa. The particular taxa that dominated the diets of each of these species was mainly related to the snout length and mouth gape of those species. However, other characteristics, such as the microhabitat to which they are adapted and their swimming ability, are also likely to exert an influence on the types of prey that are ingested by these syngnathids. The possession of a relatively long snout enables syngnathids to attack prey from a greater distance and with greater speed compared to those species with relatively short snouts. This accounts for the fact that long-snouted species, such as common seadragon Phyllopteryx taeniolatus and S. argus, fed mainly on mobile prey like mysids or planktonic copepods, whereas short-snouted species, such as H. breviceps and pug-nosed pipefish Pugnaso curtirostris ingested more sedentary prey, such as amphipods. The fact that S. argus and S. nigra ingested mainly copepods while western crested pipefish Mitotichthys meraculus, P. taeniolatus and V. poecilolaemus fed mainly on mysids could be related to the small gapes of the former species. The consumption of mainly sedentary prey by the West Australian seahorse Hippocmnpus subelongatus, which has a snout of similar length to M. meraculus, could be related to the fact that seahorses are poor swimmers and rarely forage in open water where mobile prey are typically found. Since the gut contents of S. argus and S. nigra were greatest and least digested when the sun was directly overhead, these species fed at a time when their ability to visually detect prey would have been greatest. The dietary compositions and breadths of H. breviceps, Macleay's crested pipefish Histiogamphelus cristatus, P. curtirostris, S. argus, S. nigra and V. poecilolaemus exhibited only small or moderate size-related changes. For example, although all sizes of S. argus fed mostly on calanoid and cyclopoid copepods, the smallest individuals did ingest relatively greater amounts of harpacticoid copepods. However, similar-sized S. argus ingested fewer calanoid but more cyclopoid copepods in P. sinuosa than in P. coriacea, whereas similar-sized S. nigra ate fewer calanoid and more harpacticoid copepods in the deep habitat than in P. coriacea. These habitat-related differences in dietary composition could reflect differences in the prey communities and/or the ability of syngnathids to capture prey in different habitat types. Stigmatopora argus and S. nigra sexually matured at ca 65 and 30 mm snout­vent length, respectively, and monthly samples collected during 1997 and 1998 showed that most mature-sized individuals of both species were sexually active in all months. However, a proportion of the smallest mature-sized S. argus were not sexually active between February and September and the clutches and broods of the larger individuals that were breeding during these months tended to be smaller than at other times of the year. These trends probably accounted for the densities of S. argus being lower in the winter and spring of 1997 than in summer 1997/98. Female S. nigra invested more in ovarian mass than S. argus (ca 8 vs 5% of body weight, respectively), and produced larger clutches of eggs with a smaller diameter (ca 118 and 1 mm vs 36 and 2 mm, respectively). As post-brood S. argus were markedly larger than S. nigra (ca 30 vs 10 mm standard length) and would thus more rapidly attain settlement size, S. nigra may have a longer planktonic phase. The greater fecundity of S. nigra may thus enable more young of this species to survive the increased mortality that would be associated with such a longer planktonic phase. Evidence also suggests that both S. argus and S. nigra are sex-role reversed and have sequentially polyandrous mating systems